The present invention relates to a pulse laser irradiation apparatus for a coated metal sheet whose top and tail surfaces are subjected to such as a Zn-plating treatment for a rust-proof effect. A typical plated steel sheet is defined by G3302 of Japanese Industrial Standards (JIS). A thickness of the steel sheet is in the range from 1.6 mm to 6.0 mm in the case where a heat-rolled plate, and is in the range from 0.11 mm to 3.2 mm in the case where a cold rolled plate. Such a kind of Zn-plated steel sheet is widely used in various industrial fields such as an automotive field, a general electric field, a light industrial field, a heavy industrial fields and the like.
The techniques for welding Zn-plated steel plates together will now be described in more detail. In a welded example as shown in FIG. 13(a), two steel plates 2-1 and 2-2 each having Zn-platings 3 on both sides are overlapped with each other. In another example as shown in FIG. 13(b), a single steel plate 2a having Zn platings is bent back to form a double folded structure.
In an example of a three layer welding as shown in FIG. 14(a), three steel plates 2-1, 2-2 and 2-3 each having Zn-platings 3 on both sides are laminated. In FIG. 14(b), a single steel plate 2b having Zn-plating on both sides is folded and another steel plate 2-1 having Zn-platings on both sides is inserted into a recess defined by the folded steel plate 2b.
In the same way, a plural layer welding may be carried out for four or more layers as shown in FIGS. 15(a) and (b).
A purpose of a semi-penetration welding in an overlap irradiating is to keep an outer appearance quality of the surface steel plates while insuring the bond strength at a suitable level. In other words, as shown in FIG. 14(b), it is unnecessary to carry out any surface finishing treatment after the welding, and it is possible to keep the outer surface quality in the same condition as the original steel plate surface.
Also, in the case as shown in FIG. 14(a), it is possible to keep the surface quality of the original steel plate surface only with a minimum surface finishing treatment. If, for instance, a grinding work or the like is effected on the Zn-plated steel plates as a surface treatment, a Zn-plate layer on the surface is ground away so that the rust-proof effect of the Zn-plating treatment is remarkably degraded.
In general, known methods for laser-welding of Zn-plated steel plates are categorized into, for example, a method using a continuous wave form generation type laser(which method will be hereinafter referred to as a CW method)and a method using a pulse generation type laser (which method will be hereinafter simply referred to as a pulse laser method). These methods will be explained.
(1) An example of the method using a CW type laser is an overlap welding with, for example, a CW type CO.sub.2 laser. In the CW type laser, key holes and a laser induction plasma are continuously maintained during the irradiating operation. As a result, although Zn-metallic vapor (a part of which is kept under a plasma state) which is generated by the laser beam irradiation would be removed effectively away from the keyholes, the laser output is excessively applied to the surface to be worked in comparison with the pulse laser method.
For this reason, a fugion-solidification part is increased, resulting in full penetration.
Also, even if the partial semi-penetration would be obtained be carefully selecting the irradiation conditions, it would be impossible to obtain good surface appearance of the steel plate due to the fact that there is anon-uniformity in gaps between the Zn-plated steel plates which are work pieces to be welded (which gaps will be hereinafter referred to as the "gap between the workpieces") and further there is a non-uniformity in a plating amount for the Zn-plated steel plates (for instance, in case of F08, the Zn-plating amount is in the range of 60 to 100g/m.sup.2 or due to the fact that an excessive heat causes distortion or warpage.
(2) On the other hand, as the method using the pulse laser, an overlap welding technique with a solid laser such as an Nd:YAG laser has been proposed. As the Nd:YAG laser, an overlap continuous welding and an overlap spot welding are well known.
The output of the pulse laser is given by the following relationship: The average output P(kW) is given: EQU P=E.multidot.f
The energy E(J) of one pulse is given: EQU E=P'.multidot.t
where P(kW) is the average output E(J) is the energy of one pulse, P'(kW) is the peak power of one pulse, i.e., the average peak power per one pulse width, t(msec) is the pulse width of one pulse, and f(Hz) is the pulse frequency.
In general, the irradiating volume and penetration depth relative to a predetermined welding speed in the pulse laser irradiating will mainly depend upon the pulse energy. The energy(determined by the average ouput and the frequency) of one pulse to be needed to obtain a desired penetration depth is shown in, for example. FIG. 16 in which, for example, the pulse time, the frequency and the average output are selected at optimum levels for the working conditions even in the same curve for the pulse irradiating. If the average output is kept constant, the penetration depth is increased in accordance with the pulse laser method in comparison with the CW laser method.
A pulse wave form for the pulse energy will be explained.
In a rectangular wave form as shown in FIG. 17, the peak power P(kW) is kept substantially unchanged within a pulse width t. In an integrated wave form as shown in FIG. 18, the peak power P is changed within the pulse width t(msec). FIG. 19 shows an example the wave form overlapped with the CW laser type. In these examples, the desired penetration depth is determined basically by the pulse energy of the above-described rectangular wave.
Also, in case of metals whose surface materials are likely to be blown by the pulse laser irradiation. It is possible to obtain a good welding bead by selecting the pulse energy density (i.e., peak power density).
The pulse energy and the pulse energy density (i.e., peak power density) needed to obtain a desired penetration depth relative to a plate thickness of the Zn-plated steel plate and a predetermined welding speed may readily be obtained in a well known method.
Also, in the laser irradiating process. If fumes or sputters would be stuck to lenses of an optical system. The latter would be damaged. In order to avoid this, various fume or sputter preventing methods have been proposed. Methods for protecting the optical system from the sputters and fumes are categorized into a method using a protective glass plate in front of the work lens, a method for splashing the sputters with compressed air from nozzles, and a hybrid method thereof.
However, the above-proposed methods still suffer from the following problems. Namely, in a general Nd:YAG laser pulse laser, the laser irradiation by the pulse oscillation or generation would cause key holes and laser induced plasma to occur intermittently. For this reason, the plated metal vapor (whose part is kept under a plasmatic state) or crushed organic vapor in case of organic material coting generated by the laser irradiation has to be effectively removed for every one pulse.
However, only with a single kind of rectangular pulse, it is impossible to remove the plated metal vapor or clushed organic vapor. As a result, there would be welding faults by the entrainment of the vapor into the molten material (which faults will be referred to as blow holes). Otherwise, the molten metal is splashed by the vapor pressure resulting in bond faults. In any case, the welding strength would be considerably degraded in addition to the faulty welding outer appearance such as blow holes and sputter splash.
In many cases, the sputter splash causes the optical systems to be damaged to increase a load imposed on the maintenance. Furthermore, in the case where there are gaps between the steel plates, in addition to the foregoing phenomena, the plated metallic vapor would escape through the gaps to the outside so that the welded metal is splashed or dispersed between the steel plates, resulting in welding faults. However, in some cases, if a gap between the workpieces is suppressed in a predetermined range, since the plated metal vapor will effectively escape from the gap, good weld beads may be obtained.
Nevertheless, in a field of an industrial application, it is very difficult to maintain the gap between the workpieces in the predetermined range. Also, turning to the protection of the optical systems, in accordance with the conventional methods, since the sputters are splashed with large momentums, it is impossible to completely remove the sputters with the compression air, resulting in adhesion of sputters on the protective glass or damages of the optical systems.
It is also impossible to completely remove fumes (metal particles). The fumes would stick on optical components such as a protective glass or a parabolic mirror. This would increase cost for the optical components such as a protective glass. In addition, the replacement of the optical components is time-consuming.
Accordingly, a primary object of the present invention is to provide a pulse laser irradiation method for irradiating a pulse laser beam onto plated steel plates, and more particularly a pulse laser irradiation apparatus by which weld beads having a good weld outer appearance with little faults such as blow holes and a small splash such as sputters and the like with a sufficient shearing force may be obtained by semi-penetration in an overlap spot and an overlap continuous weld (lap seam weld) for plated steel plates.